Valves and other types of flow control systems are commonly used in a variety of different types of applications for controlling the flow of various fluid materials through flow lines. Such different types of valves include needle valves, spool valves, butterfly and pinch valves, among others. Generally, such valves will include a body or housing through which a flow passage is defined and having a valve element mounted along the flow passage and which is movable between an open and a closed position for opening and closing the flow passage to restrict the fluid flow therethrough. For example, most pinch valves generally are mounted along a flexible flow line and include one or more movable pinch jaws that are moved or closed together to engage the flow line therebetween to pinch or shut off the fluid flow through the flow line.
In the past, most conventional valves typically have been mechanically actuated or, more recently, electromechanically actuated such as by use of a solenoid. Such solenoid actuated valves are, however, generally limited to being on/off type valves in which the valve elements are moved either to a fully open or fully closed position. In many fluid dispensing applications, such as medical or pharmaceutical applications, where control of the amount of fluid being dispensed is important and in some cases critical, however, it is desirable for the actuation of the valves to be controlled or regulated so that the valves can be closed by a desired amount to precisely and proportionally regulate the fluid flow therethrough as desired. In addition, such electromechanically actuated valves generally are relatively cumbersome and expensive and are not readily replaceable or disposable, or require their actuator, i.e., a solenoid, to first be disconnected and removed before discarding the valve assembly.
More recently, other types of actuators have been issued for controlling the actuation of flow control valves. For example, U.S. Pat. No. 4,973,024 of Homma discloses the use of a shape memory alloy (SMA) for moving a valve element between open and closed positions to open and close a valve port. Shape memory alloys generally are metal alloy wires that tend to contract in response to heating of the alloy by application of an electric current therethrough. Thereafter, as the alloys cool back to room temperature, they are easily stretched back to their normal length. The contracting force of the alloys further exerts significant pulling force and is used in Homma for moving a valve element out of engagement with a valve port to open the valve port.
One problem with such shape memory alloys is that the strength or force exerted by the alloy is dependent upon the size of the alloy wire used such that a thicker alloy wire is required for greater compressive strength. However, as the size of the alloy (i.e., the thickness of the wire) is increased, the reaction time of the alloy, and thus the timing of the opening and closing of the valve, generally decreases without application of significantly increased current, potentially impairing the performance of the valve. Thus, such shape memory alloys may not be practical for use in some applications where precise proportional control of a fluid flow is required.
Accordingly, it can be seen that a need exists for a proportional flow control valve that is able to address these and other related and unrelated problems in the art.